The present invention relates generally to inkjet printing devices, and more particularly to an inkjet printhead drop generator that utilizes a high resistance heater resistor structure with current crowding reduction.
The art of inkjet printing technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines successfully employ inkjet technology for producing hard copy printed output. The basics of the technology has been disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Inkjet devices have also been described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices (R. C. Durbeck and S. Sherr, ed., Academic Press, San Diego, 1988, chapter 13).
A thermal inkjet printer for inkjet printing typically includes one or more translationally reciprocating print cartridges in which small drops of ink are formed and ejected by a drop generator towards a medium upon which it is desired to place alphanumeric characters, graphics, or images. Such cartridges typically include a printhead having an orifice member or plate that has a plurality of small nozzles through which the ink drops are ejected. Beneath the nozzles are ink firing chambers, enclosures in which ink resides prior to ejection by an ink ejector through a nozzle. Ink is supplied to the ink firing chambers through ink channels that are in fluid communication with an ink supply, which may be contained in a reservoir portion of the print cartridge or in a separate ink container spaced apart from the printhead.
Ejection of an ink drop through a nozzle employed in a thermal inkjet printer is accomplished by quickly heating the volume of ink residing within the ink firing chamber with a selectively energizing electrical pulse to a heater resistor positioned in the ink firing chamber. At the commencement of the heat energy output from the heater resistor, an ink vapor bubble nucleates at sites on thesurface of the heater resistor or its protective layers.
The rapid expansion of the ink vapor bubble forces the liquid ink through the nozzle. Once the electrical pulse ends and ink is ejected, the ink firing chamber, refills with ink from the ink channel and ink supply.
The electrical energy required to eject an ink drop of a given volume is referred to as "turn-on energy". The turn-on energy is a sufficient amount of energy to overcome thermal and mechanical inefficiencies of the ejection process and to form a vapor bubble having sufficient size to eject a predetermined amount of ink through the printhead nozzle. Following removal of electrical power from the heater resistor, the vapor bubble collapses in the firing chamber in a small but violent way. Components within the printhead in the vicinity of the vapor bubble collapse are susceptible to fluid mechanical stresses (cavitation) as the vapor bubble collapses, allowing ink to crash into the ink firing chamber components. The heater resistor is particularly susceptible to damage from cavitation. A protective layer, comprised of one or more sublayers, is typically disposed over the resistor and adjacent structures to protect the resistor from cavitation and from chemical attack by the ink. The protective sublayer in contact with the ink is a thin hard cavitation layer that provides protection from the cavitation wear of the collapsing ink. Another sublayer, a passivation layer, is typically placed between the cavitation layer and the heater resistor and associated structures to provide protection from chemical attack. Thermal inkjet ink is chemically reactive, and prolonged exposure of the heater resistor and its electrical interconnections to the ink will result in a chemical attack upon the heater resistor and electrical conductors. The protection sublayers, however, tend to increase the turn-on energy required for ejecting drops of a given size. Additional efforts to protect the heater resistor from cavitation and attack include separating the heater resistor into several parts and leaving a center zone (upon which a majority of the cavitation energy concentrates in a top firing thermal inkjet firing chamber) free of resistive material.
The heater resistor of a conventional inkjet printhead utilizes a thin film resistive material disposed on an oxide layer of a semiconductor substrate. Electrical conductors are patterned onto the oxide layer and provide an electrical path to and from each thin film heater resistor. Since the number of electrical conductors can become large when a large number of heater resistors are employed in a high density (high DPI--dots per inch) printhead, various multiplexing techniques have been introduced to reduce the number of conductors needed to connect the heater resistors to circuitry disposed in the printer. See, for example, U.S. Pat. No. 5,541,629 "Printhead with Reduced Interconnections to a Printer" and U.S. Pat. No. 5,134,425, "Ohmic Heating Matrix". Each electrical conductor, despite its good conductivity, imparts an undesirable amount of resistance in the path of the heater resistor. This undesirable parasitic resistance dissipates a portion of the electrical power which otherwise would be available to the heater resistor. If the heater resistance is low, the magnitude of the current drawn to nucleate the ink vapor bubble will be relatively large and the amount of energy wasted in the parasitic resistance of the electrical conductors will be significant. That is, if the ratio of resistances between that of the heater resistor and the parasitic resistance of the electrical conductors (and other components) is too small, the efficiency of the printhead suffers with the wasted energy.
The ability of a material to resist the flow of electricity is a property called resistively. Resistively is a function of the material used to make the resistor and does not depend upon the geometry of the resistor of the thickness of the resistive film used to form the resistor. Resistively is related to resistance by: EQU R=.rho.L/A
where R=resistance (Ohms); .SIGMA.=resistively (Ohm-cm); L=length of resistor; and A=cross sectional area of resistor. For thin film resistors typically used in thermal inkjet printing applications, a property commonly known as sheet resistance (R.sub.sheet) is commonly used in analysis and design of heater resistors. Sheet resistance is the resistively divided by the thickness of the film resistor, and resistance is related to sheet resistance by: EQU R=R.sub.sheet (L/W)
where L=length of the resistive material and W=width of the resistive material. Thus, resistance of a thin film resistor of a given material and of a fixed film thickness is a simple calculation of length and width for rectangular and square geometries.
Most of the thermal inkjet printers available today use heater resistors that are roughly of a square shape and have a resistance of 35 to 40 Ohms. If it were possible to use resistors with higher values of resistance, the energy needed to nucleate an ink vapor bubble would be transmitted to the thin film heater resistor at a higher voltage and lower current. The energy wasted in the parasitic resistances would be reduced and the power supply that provides the power to the heater resistors could be made smaller and less expensive. Realization of the higher values of resistance, however, may increase the current density despite the overall current reduction. High current density can reduce the life of electronic circuits by creating localized elevated temperatures and by generating high electric field strengths that induce electromigration in materials. Moreover, in applications where the current is switched on and off, such as in thermal inkjet heater resistors, extreme thermal cycling produces expansion and contraction, which results in fatigue failures.